A number of published papers employ an ultraviolet-induced T4 reversion system to explore aspects of UV mutagenesis. Concern about a possible artefact in the measurements, plate reactivation, prompted measurements to determine the extent to which the putative reversion response is independent of the plating density of irradiated particles. All preliminary experiments indicate that most of the claimed reversion is a reactivation artefact. The experiments will now be repeated under conditions which more exactly reproduce those described in the published papers.